Pentachlorophenol (PCP) is a highly toxic material. It is hydrophobic and is recalcitrant to biological degradation. Improper use of PCP has caused serious soil and groundwater contamination problems worldwide, and posed threat to water reclamation processes. The objective of this study was to understand the effects of ozonation on the distributions of ring intermediates during the initial stage of oxidation reactions. Experiments were designed and conducted in a homogeneous condition. Molar ratios of ozone to PCP were controlled in the range from 0.05 to 1.3 at pH 6 to examine dosage effects. Three major ring intermediates quantified were tetrachloro-p-hydroquinone (TCHQ), tetrachloro-p-benzoquinone (TCBQ), and tetrachlorocatechol (TCCA). Transient distributions of these three compounds, together with chloride ions, were monitored. A degradation pathway was proposed and a set of kinetic equations was written accordingly. Rate constants in the kinetic model were obtained by optimizing against experimental data. Ozone dosage was found to be a factor controlling byproducts distributions. Finally, the dynamic sensitivities of rate constants were evaluated for each compound to understand their specific behaviors. (C) 2011 Elsevier B.V. All rights reserved.